In accordance with theories known since the work of Abbe and Rayleigh, diffraction is the cause for the principal limitation of the resolving power achieveable with optical systems, where the resolving power is defined by the wavelength of the radiation and by the numerical aperture of the system employed.
The limitation mentioned above also applies to optical memories where bit density is limited by the optical resolution, which in turn is limited by the smallest diameter to which a laser beam can be focussed. Since this approximately corresponds to one wavelength, the theoretical storage density using a 600 nm laser beam is of the order of 10.sup.8 bit/cm.sup.2.
Optical memories are well known. Reference is made to Taschenbuch Elektrontechnik, Bd. 4, Munchen 1979, p. 638. A narrowly focussed laser beam is directed onto a picture point of the storage medium in accordance with a specified address, in order to write, read or cancel information. For writing, the information may, e.g., be represented by the modulation superimposed on the laser beam for changing the physical properties of the storage medium. For reading, the laser beam as reflected by, or passing through, the addressed picture point of the storage medium is analyzed by a detector which issues an output signal representative of the information stored. A typical arrangement of an optical disk memory is shown in FIG. 18 of the paper by C. Harder, K. Y. Lan and A. Yariv "Bistability and Pulsation in Semiconductor Lasers with Inhomogeneous Current Injection", IEEE J. QE-18, No. 9 (1982), pp. 1351-1361.
An improvement regarding the storage density of optical memories is only possible if the laser beam can be restricted to an area smaller than is feasible with conventional means. It is one aspect of the present invention to describe an improved optical aperture the diameter of which is smaller than one wavelength of the laser light used. Apertures of this size have so far not been disclosed (other than in the aforementioned related application) as their manufacture has not been envisaged before, and generally because they are not useful in imaging optics.
The present invention contemplates the creation of an aperture of submicron size by means of a sharp crystal covered with an opaque coating and with said coating removed at the tip so as to expose a tiny area of the crystal through which the light can pass. A crystal used as an enlarging lens is known from an article by T. S. Fitzgerald entitled "Self-Image and Enlarging Lens", IBM Technical Disclosure Bulletin, Vol. 18, No. 12, May 1976, p. 4174, in which an inverted pyramid-shaped lens is used for enlarging an image recorded on photographic film and displaying the enlarged image on its frosted base surface. However, this crystal does not have a metal coating, nor is its optical behaviour relevant to the waveguide of the present invention.